"Sources tell us that Nokia is developing a Linux-based replacement for its S40 phones, called Meltemi. The news was leaked, accurately, by the Wall Street Journal last week. Now we can confirm it. The thinking is that a Linux-based replacement for S40 will allow developers to tap into proven development tools - and Qt. The April memo referred to Meltemi as a platform for 'rich Featurephones' and stated that development will be centered in Ulm, Germany. There's no U-turn, however. Meltemi had been long-been touted as a richer successor to S40. Windows phones will occupy the budget smartphone segment, not Linux."

Well, there is no real definition and real feature phones are probably dead in the long run .. so really cheap phones are also likely to gain the ability to install apps (most of them can now, but mostly just java apps)

In the future the differentians will be more on price.
Theoretically you just need this hardware for a great and smooth smartphone OS:
SOC with 500 Mhz and 128 mb RAM and 1 GB of storage.
Those specs might be way above what S40 has now on average, but 2014 it will be low end.

Not having a VM like Android and Windows Mobile Phone 7.5 Series Phones helps a lot with reducing requirements.
Linux + KMS + Wayland + Qt5 could be fairly lightweight.